Process for producing fine acicular gamma ferric oxide crystals

ABSTRACT

Fine goethite crystals (alpha-FeO.OH, ferric oxide hydrate) are prepared by mixing together a ferrous salt such as the sulfate or chloride and an alkali hydroxide to form a hydroxide precipitate as a dispersion, blowing oxygen through the dispersion at 20*60*C. to convert ferrous hydroxide particles to goethite crystals, discontinuing blowing, boiling the dispersion to perfect crystallization of goethite crystals, and recovering the goethite crystals by filtering, washing and drying. The goethite crystals so produced are only about 0.3 microns long, and can be transformed into magnetic ferric oxide crystals which have exceptionally good properties for magnetic recording tapes, particularly for improving the background noise dynamic so that there is less noise during intermittent no-signal periods that may occur during sound recording.

United States Patent 1191 Pingaud Oct. 22, 1974 [54] PROCESS FORPRODUCING FINE FOREIGN PATENTS OR APPLICATIONS ACICULAR GAMMA FERRICOXIDE 505 751 9/1954 Canada 4 3/ 68 CRYSTALS Inventor: Bernard-Jean gVincermes. Primary Examiner-Herbert T. Carter France Attorney, Agent, orFirm-Bernard D. Wiese [73] Assignee: Eastman Kodak Company,

Rochester, NY. [57] ABSTRACT 22 Fil d; Jam 3 972 Fine gocthite crystals(alpha-FeOOH, ferric oxide hydratc) are prepared by mixing together aferrous salt [21] Appl- N05 222,318 such as the sulfate or chloride andan alkali hydroxide Related .s Application Data to form a hydroxideprecipitate as a dispersion, blow- [63] Continuation Of Ser. NO, 36,385,May 11, 1970, mg Oxygenthmugh t 9 to abandmed vert ferrous hydroxIdeparticles to goethlte crystals,

discontinuing blowing, boiling the dispersion to per- 52 us. (:1.423/634, 252/6256 fact crystellization of goethite crystals: andrecovering 51 Int. c1 C01g 49/06 the goethlte crystals by filteringwashmg and drying- [58] Field of Search 423/634; 75/103; 106/304; Thegoethite crystals so produced are only about 0.3 252/6256 microns long,and can be transformed into magnetic ferric oxide crystals which haveexceptionally good [56] References Cited properties for magneticrecording tapes, particularly UNITEDSTATES PATENTS for improving thebackground noise dynamic so that a 558302 6/1951 Marcot 6t al. 423/268there is less noise dining intermittent. nosignal Periods 3107519191/1963 Gruber et al. 423/634 that may Occur durmg Sound recordmg-3,252,758 5/1966 Hoch et al7 423/634 9 Claims N0 Drawings 4 3,288,56311/1966 Klomp et al. 423/634 PROCESS FOR PRODUCING FINE ACICULAR GAMMAFERRIC OXIDE CRYSTALS This is a continuation of application Ser. No.36,385, filed May ll, 1970 and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to anew process for the preparation of fine magneticgamma-ferric oxide usable in the preparation of magnetic tapes.

2. The Prior Art Gamma-ferric oxide, or maghemite, has been known as thematerial for the production of magnetic tapes useful in the recordingand sound reproduction of images or signals. It is known that theacoustical characteristics of these tapes are strongly influenced by thesize and the shape of the magnetic gamma-ferric oxide particles. Asconcerns the shape, the acicular shape has been considered to be themost advantageous, and, preferably, the length of each particle is equalto approximately from three to eight times its diameter. As concerns thesize, it is known that when the particles are small, one can obtaintapes which display particular properties, very desirable for certainuses, such as a good no-signal background noise. The no-signal noise isthe noise which appears during the reading of a magnetic tape which wassubmitted only to the premagnetization field without sound recording orother signals. This noise, which is particularly disturbing during thelistening of recordings with intermittent silences, is related to thesize of the crystals: the smaller the size of the crystals the more theno-signal noise is moved toward frequencies that become higher andhigher, i.e., less and less audible. The fine crystals are alsoadvantageous for obtaining a better frequency response thanks to theincrease in the level of clear frequencies.

The ferromagnetic compound that is used foi the manufacture of magnetictapes is the acicular iron sesquioxide, gamma-Fe O or maghemite. Thiscompound is obtained from a non-magnetic hydrated ferric oxide, goethitealpha-Fe0.0H. The first step-to obtain maghemite is to dehydrategoethite in order to obtain hematite, followed by a reduction tomagnetite, then to effeet a careful oxidation so as to obtain themagnetic maghemite. i

The diagram of the reactions is as follows:

2 alpha-Fc0.0H alpha Fe O H O (goethite) (hematite) 3 alpha-H3 H 2 Fc;,OH O (magnetite) 2 [-1 0 A: 0., 3 gamma Fe O (maghemitc) For thepreparation of the crystals of goethite numerous processes have beenused up until now. They consist particularly in precipitating ferroushydroxide from an alkaline hydroxide solution and a ferrous saltsolution, then in oxidizing the precipitated hydroxide. However, theprocess that is effected in an acid medium (excess of a ferroussolution) or basic (excess of an alkaline solution) yields crystalsthesize of which (often in the range of 1 micron) are too large foryielding magnetic oxide crystals having good properties, as mentionedpreviously. When the precipitation of ferrous hydroxide occurs fromstoichiometric quantities of reagents, one obtains cubic magnetite mixedwith some kaline medium, e.g., in the presence of a strong excess ofalkaline hydroxide (approximately lOO percent) the crystals still havea'length in the range of l micron.

SUMMARY OF THE lNVENTlON In accordance with the present invention thereis provided a process for the preparation of goethite particles(alpha-FeOOH) having a length in the order of 0.4 microns, with no traceof other types of iron oxide, such as lepidocrocite and magnetite. Theseparticles are sub jected to dehydration, reduction and oxidation toconacicular particles of goethite. One obtains in an acid vert them togamma ferric oxide (maghemite) which is very useful in magneticrecording.

The process according to the invention for the preparation of acicularferric oxide hydrate, alpha-Fe0.0l-l in the form of fine crystals,consists in precipitating at a temperature below C a ferrous hydroxidefrom a ferrous salt solution and an alkaline solution used inexcess withrespect to the stoichiometric quantity that is necessary. The resultinghydroxide dispersion is oxidized at a temperature approximately of from20C to 60C, then is filtered, washed and dried.

The process is characterized in that one disperses, in the absence ofany oxidizing agent whatsoever, the solution of ferrous salt in thealkaline solution in such a way that there is practically no localizedexcess of ferrous salt and that the alpha-Fe0.0H concentration in thefinal dispersion that is obtained is below 15 g/l, that the finalconcentration of dissolved alkaline hydroxide is below 60 g/l, and,after oxidation, the dispersion is brought to boiling in order tocomplete the crystallization.

According to the invention, one adds, while stirring vigrouously (in theabsence of any oxidizing agent whatsoever) a dilute aqueous solution ofa ferrous salt (such as ferrous sulfate heptahydrate) to a solution ofan alkali metal hydroxide (such as sodium hydroxide) in an excess ofapproximately 200 percent with respect to the stoichiometricallynecessary quantity, at a temperature below 40C and at a pH substantiallyequal to 14 so as to form a ferrous hydroxide precipitate. One thenslowly bubbles in through the dispersion a stream comprising gaseousoxygen, such as pure oxygen or air, at a temperature close to roomtemperature in order to oxidize and transform the ferrous hydroxideparticles into goethite crystals. The oxygen is then discontinued andthe dispersion is then heated to boiling for several hours in order tocomplete the crystallization, after which the crystals are filtered,washed and dried. Fine acicular goethite crystals are obtained. havingan ap proximate length of 0.3 microns, which can then be transformedinto magnetic ferric oxide by means of usual processes, e.g., theprocess described in British 'Pat. No. 640,438 and in Phys. Chem. Solids23 p.

The precipitation is done, while suitably stirring, by introducing theferrous salt solution into the alkaline hydroxide solution; or else onemay incorporate the excess alkaline hydroxide solution in the reactioncontainer, then introduce at the same time the ferrous salt solution andthe alkaline hydroxide solution, these processes making it possible toavoid any localized excess of ferrous salt.

As ferrous salt, one may use ferrous sulfate heptahydrate, as well asother salts such as FeCl- 4H O. Sodium hydroxide may be used as well aspotassium hydroxide.

The ferrous salt concentration may vary, but it must be such at the endof' the reaction that the alpha- Fe0.0H concentration in suspension isbelow g/l of solution. The concentration of the alkali hydroxidesolution at the beginning of the reaction may also vary, but it must besuch that the concentration of the dissolved alkali hydroxide afterprecipitation is below 60 g/l of solution, e.g. the initialconcentration of caustic soda may be 60 g/l.

It is necessary that the precipitation occur in the absenceof anyoxidation whatsoever, in order to avoid at that state the formation ofheterogeneous crystals which would later lead to crystals the magneticcharacteristics of which would be different from the desired optimum.The precipitation may be made within a period of time that may vary. Itis preferably, done slowly so as to avoid a localized excess of Fe(OH)The duration is, advantageously, several hours and may last up toapproximately 3 hours.

The temperature of the precipitation may vary. It is preferably below40C, since for example, for an excess of 200 percent of sodiumhydroxide, above 40C one obtains a mixture of magnetite and goethite,then at a still higher temperature, magnetite alone. The temperature ofthe precipitation is advangeously C.

The oxidation is done slowly, at least over a period of 24 hours and,preferably, over several days, e.g., 3 days to 5 days, in order that thecrystals are uniform. The temperature is about room temperature. With ahigher temperature, one obtains larger crystals. The output of air oroxygenis low, in the range of 5 to l/hr. per liter of solution,,in orderto have a slow oxidation. In order to obtain homogenous crystals, thegrowth of the seed crystals must be carefully controlled, which requiresa constant rate of oxidation; this can only be accomplished by aprogressive introduction of air in the suspension while it is beingstirred because the high density of the mixture does not favor in thesolution the solid-gas" contacts. In order to effect the dispersion andthe oxidation, one may use any apparatus which will make it possible todisperse a fluid containing a large number of particles in a liquid.

After the oxidation, the suspension is boiled for 6 to 8 hours. Thismakes it possible to complete the crystallization, i.e., to obtain anarrangement of the crystal lattice that is desirable for obtaining goodmagnetic properties.

When alpha-Fe0.0H ferric oxide crystals are prepared under otherconditions, one obtains larger acicular particles, e.g. when the ironconcentration in the suspension is above 10 g/l (or the concentration inalpha-Fe0.0H above 16 g/l), the concentration of the dissolved alkalinehydroxide above 60 g/l, the air flow 60 l/hr. per liter of solution, thetemperature being above C. I

THE PREFERRED EMBODIMENTS The following examples illustrate theinvention:

EXAMPLE I One introduces in a 25 liter container, 12 liters of anaqueous sodium hydroxide solution containing 720 g of NaOH. Whilestirring the solution, one adds, in 3 hours, under a current of nitrogenand in a homogenous manner, 6 liters of a solution containing 750 g ofFeSO, 7H O, the temperature being 25C. Once the precipitation iscompleted, one bubbles in compressed air through the solution at therate of i6 l/hr per liter of so lution for 5 days, the temperature being25C. Goethite crystals are formed. The air is stopped and the reactionmixture is. then kept boiling for 6 to 8 hours, so as to complete thecrystallization. The product is filtered, washed and dried. The size ofthe needles is in the order of 0.2 microns to 0.3 microns.

EXAMPLE ll In the preceding example, the ferrous hydroxide precipitationwas effected in the presence of an excess of alkaline solution,obviously greater at the beginning than at the end of the reaction. Byadding simultaneously, e.g., by means of an intermittent injection pump,the two solutions into the reactor, it is possible to maintain thisexcess constant during the course of the reaction. This techniquefacilitates the obtaining of a homogenous substance; it makes itpossible, on the other hand, to have available one new method ofoperation: the time it takes to add the reagents, in order to controlthe size of the ferric oxide crystals.

In a 2 liter container, one introduces 300 ml of a sodium hydroxidesolution containing 25.6 g of NaOH. Over a period of 2 hours, oneintroduces simultaneously 300 ml of a ferrous sulfate solutioncontaining 37.5 g of FeSO, 7H O, and 300 ml of a sodium hydroxidesolution containing 10.4 g of NaOH. The mixture is stirred and theprecipitated ferrous hydroxide is oxidized with an air flow of 30 l/hrper liter of solution, for 3 days. The temperature during precipitationand oxidation is kept at 25C. One obtains fine and uniform goethitecrystals.

EXAMPLE Ill The same procedure as above is effected, this time byintroducing the reagents within 3 hours. One observes, in this case, asubstantial increase in the length of the goethite needles.

The precipitation and the oxidation of the ferrous hydroxide may also beeffected with a solution of potassium hydroxide and ferrous sulfate,having the same concentrations. One may also introduce the ferroussulfate in the solid state into the alkaline solution in order to avoidthe instability of this compound in solution.

EXAMPLE IV After treatment of the alpha-FeOOH ferric oxide crystals(according to the process described in British Pat. No. 640,438) inorder to obtain magnetic gamma- Fe O ferric oxide crystals, one mayprepare a magnetic tape in the usual manner. The obtained results with amagnetic tape comprising oxide crystals prepared according to theinvention are compared with those obtained with a magnetic tapecontaining conventional oxide crystals having from 0.7 microns to 0.8microns The invention has been described in detail with particularreference to preferred embodiments thereof, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

ll) We claim:

1. A process for preparing fine acicular crystals of gamma ferric oxidefor magnetic recording comprising reacting under non-oxidizingconditions, at a temperature below 60C, an aqueous solution of a ferroussalt with an aqueous solution of a stoichiometric excess ofan alkalinehydroxide to form an aqueous dispersion of ferrous hydroxide particles;

introducing oxygen into said dispersion at a temperature of -60C for aperiod of time sufficient to convert said ferrous hydroxide particles tocrystals of alpha FeO'OH, the concentration of ferrous salt in saidaqueous solution being such that the concentration of said alpha FeO-OHformed in said dispersion is less than 15 grams per liter;

discontinuing introduction of said oxygen;

boiling said dispersion to obtain further crystallization of said alphaFeO'OH crystals;

recovering said alpha FeO'OH crystals from said dis- 3 persion; and

subjecting said crystals to dehydration, reduction and 6 oxidation toform gamma ferric oxide crystals.

2. A process in accordance with claim 1 in which said alkaline hydroxideis an alkali metal hydroxide.

3. A process in accordance with claim Zwherein the introduction ofoxygen into said dispersion is for at least 24 hours, and the boiling ofsaid dispersion is for at least 6 hours.

4. A process in accordance with claim 2, wherein said ferrous salt isFeSO, 7H O, and said alkali metal hydroxide is sodium hydroxide.

5. A process in accordance with claim 2 wherein said reacting step isconducted at a temperature below about 40C.

6. A process in accordance with claim 2 wherein the concentration ofalkali metal hydroxide in said alkali metal hydroxide solution is lessthan grams per liter.

7. A process in accordance with claim 2 wherein said oxygen isintroduced as air at a rate of about 5-30 liters/hr per liter ofdispersion for a period of 1-5 days.

8. A process in accordance with claim 6 wherein said aqueous solution ofa ferrous salt is added to said aqueous solution of an alkali metalhydroxide.

9. A process in accordance with claim 6 wherein an aqueous solution ofsaid alkali metal hydroxide is introduced into a vessel, and then saidferrous salt solution and an additional quantity of an aqueous solutionof said alkali metal hydroxide are introduced simultaneously into saidvessel.

l I l Patent No.

Inventor(s) UNITED STATES PATENT OFFICE Bernard-Jean Pingaud It iscertified that error appears in the abovefii'deptified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the abstraet, line l, insert --ferrous-- before "hydroxide". I

Column 1, line #1, delete "alpha-Fe0.0H" and substitute therefor--(alphs-Fe0.0H)

Column 2, line 2 L, after "Fe0.0H" insert 1--,--.

Column 2, line 49, delete "40C" and substitute therefor --60C--.

Column 3, line 21, after "soda" insert --at the beginning of theoxidation reaction--.

Signed and sealed this 4th day of March 1975.

Attest:

RUTH c. MASON I Attesting Officer 0. MARSHALL DANN Commissioner ofPatents and Trademarks

1. A PROCESS FOR PREPARING FINE ACICULAR CRYSTALS OF GAMMA FERRIC OXIDEFOR MAGNETIC RECORDING COMPRISING REACHING UNDER NON-OXIDIZINGCONDITIONS, AT A TEMPERATURE BELOW 60*C, AN AQUEOUS SOLUTION OF AFERROUS SALT WITH AN AQUEOUS SOLUTION OF A STOICHIOMETRIC EXCESS OF ANALKALINE HYDROXIDE TO FORM AN AQUEOUS DISPERSION OF FERROUS HYDROXIDEPARTICLES; INTRODUCING OXYGEN INTO SAID DISPERSION AT A TEMPERATURE OF20*-60*C, FOR A PERIOD OF TIME SUFFICIENT TO CONVERT SAID FERROUSHYDROXIDE PARTICLES TO CRYSTALS OF ALPHA FEO.OH, THE CONCENTRATION OFFERROUS SALT IN SAID AQUEOUS SOLUTION BEING SUCH THAT THE CONCENTRATIONOF SAID ALPHA FEO.OH FORMED IN SAID DISPERSION IS LESS THAN 15 GRAMS PERLITER; DISCONTINUING INTRODUCTION OF SAID OXYGEN; BOILING SAIDDISPERSING TO OBTAIN FURTHER CRYSTALIZATION OF SAID ALPHA FEO.OHCRYSTALS; RECOVERING SAID ALPHA FEO.OH CRYSTALS FROM SAID DISPERSING;AND SUBJECTING SAID CRYSTALS TO DEHYDRATION REDUCTION AND OXIDATION FROMGAMMA FERRIC OXIDE CRYSTALS.
 2. A process in accordance with claim 1 inwhich said alkaline hydroxide is an alkali metal hydroxide.
 3. A processin accordance with claim 2 wherein the introduction of oxygen into saiddispersion is for at least 24 hours, and the boiling of said dispersionis for at least 6 hours.
 4. A process in accordance with claim 2,wherein said ferrous salt is FeSO4 . 7H2O, and said alkali metalhydroxide is sodium hydroxide.
 5. A process in accordance with claim 2wherein said reacting step is conducted at a temperature below about40*C.
 6. A process in accordance with claim 2 wherein the concentrationof alkali metal hydroxide in said alkali metal hydroxide solution isless than 60 grams per liter.
 7. A process in accordance with claim 2wherein said oxygen is introduced as air at a rate of about 5-30liters/hr per liter of dispersion for a period of 1-5 days.
 8. A processin accordance with claim 6 wherein said aqueous solution of a ferroussalt is added to said aqueous solution of an alkali metal hydroxide. 9.A process in accordance with claim 6 wherein an aqueous solution of saidalkali metal hydroxide is introduced into a vessel, and then saidferrous salt solution and an additional quantity of an aqueous solutionof said alkali metal hydroxide are introduced simultaneously into saidvessel.